1. Field of the Invention
The present invention relates to impact printers and more particularly it relates to controlling the flight time of the impact means or hammer of such impact printer by controlling the amplitude of the energy to the hammer.
2. Description of Prior Art
Impact printers which utilize a print wheel, i.e., rotating disk with characters on the periphery thereof, are well known. Several of such printers are commercially available. Rotating disk printers can be divided into categories by either focusing on how the disk rotates or by focusing on how the carrier traverses.
Focusing on how the disk rotates, such printers can be divided into a first category where the disk constantly rotates and into a second category where the motion of the disk is intermittent. In printers with a constantly rotating disk, printing takes place when the hammer strikes the rotating disk. Rotation of the disk is not stopped each time a character is printed. In printers with a disk that intermittently rotates, the disk is rotated to the desired print position and then stopped. There is no disk rotation while printing takes place.
An alternate division of disk printers can be made by focusing upon the motion of the carrier. In some printers, the traverse of the carrier is stopped each time printing takes place. In other printers, the carrier is moving at the instant when printing occurs. In both the type where the carrier is moving when printing occurs and in the type where the carrier is stopped when printing occurs, the disk may or may not be rotating at the time of printing. In some printers where the carrier is moving at a fixed speed when printing takes place, the carrier is slowed down and stopped between print positions in order to give the rotating disk time to move to the desired character.
The following are some of the issued and pending patents which show rotating disk printers:
The Willcox U.S. Pat. No. 3,461,235 issued Aug. 12, 1969 shows a disk printer with a constantly rotating disk. The carrier stops at each print position.
The Ponzano U.S. Pat. No. 3,707,214, issued Dec. 26, 1972, discloses a disk printer which has separate controls for a print wheel and its carrier. The print wheel and the carrier move by the shortest distance to the next selected position. The print wheel and the carrier stop at each print position.
The Robinson U.S. Pat. No. 3,356,199, issued Dec. 5, 1967, describes a rotating disk printer wherein the disk is constantly rotating. The type elements on the disk are in a particular sprial configuration. The carrier also moves at a constant speed which is synchronized with the motion of the disk in such a manner that the desired character can be printed in each print position.
U.S. Pat. No. 4,030,591, Martin et al., issued June 21, 1977, discloses a rotating disk printer where the carrier is moving at a variety of velocities when the printing by the firing of the print hammer takes place. Thus, the firing of the print hammer must be timed dependent on the velocity of the carrier or carriage at the particular instance.
In U.S. Pat. No. 3,858,509, issued Jan. 7, 1975, a rotating disk printing apparatus is disclosed in which the striking force applied to the hammer can be varied between "light" and "hard". However, in that patent the printing is not done on-the-fly and there is no need to coordinate the speed of the carriage and the travel time of the print hammer to insure that the position of the character to be printed is at the print impact point at the time it is caused to strike the printing medium.
U.S. Pat. No. 4,035,780, L. H. Chang, issued July 12, 1977, mentions a procedure in a printer wherein upon a failure to print, at least one retry to print is made before the apparatus is stopped for an error correction routine. This patent does not involve on-the-fly printing wherein the carrier is never stopped. In the apparatus of the patent, the carrier appears to stop at each print position. Thus, it appears to be unrelated to the problem of synchronization of time related parameters in on-the-fly printers.
Further developments with rotating disk printers covered in U.S. Pat. No. 4,189,246, M. H. Kane et al., relate to rotating disk printers in which the carrier is moving at a variety of velocities, the rotatable character disk is rotating over a variety of distances and the print hammer is driven at a variety of forces in order to achieve consistent and high print quality. Thus, the approach in the Kane, et al. patent adds a further element, i.e., variable hammer force which must be coordinated with a variable carriage velocity and variable disk rotation distance in order to achieve the desired synchronization of selected printed character with the selected carrier print position. The hammer force in the Kane, et al. patent is varied by varying the duration of the current pulse used to drive the hammer.
Thus, for many advanced impact printing operations, the impact means is driven at the variety of forces each determined by the combination of the variable escapement velocity and variable hammer force required to achieve a consistent print quality with characters of different sizes. The result is that tolerances in impact means characteristics such as flight time are exceedingly close. Any minute variation in the impact means, i.e, hammer missile flight time due to wear or other minor misfunctions can seriously impede the operation of the impact printing apparatus. Also, a failure to achieve an exact coincident engagement of the missile with the selected type element on a print wheel can do serious damage to the print wheel and other parts of the printing apparatus. Consequently, it became critical in advanced printing operations that means be provided for monitoring the flight time of impelled impact means such as missiles and that further means be provided for detecting whether the required coincident engagement of the impact means with the type element had been achieved.
Any variation in missile flight time will result in a variation in the horizontal alignment of the printed character in on-the-fly printers where printing occurs with the carrier in motion. Even more significantly and irrespective of whether printing is on-the-fly, the variation of flight time will result in a change in the impact energy which will result in a poorly printed character; it may even damage the type element being struck, particularly if a relatively small character is struck with a relatively high energy. Another problem which can be highly disruptive to the operation of impact printing equipment occurs when the impact means, i.e., missile, fails to achieve coincident engagement with a selected type element on the print wheel. This can result in a bent or damaged wheel which may be hung-up on the missile. In such a situation, when the print wheel is subsequently rotated in the selection cycle, the movement can destroy the hung-up print wheel and damage the hammer mechanism.
Early attempts were made to monitor missile flight time by using impact sensing means such as contact point or piezoelectric sensing means on the printer platen or in the missile to determine the exact time of physical contact with the platen. With such approaches, by timing the period from when the missile firing pulse is initiated until contact with the platen is directly sensed, flight time may be determined. These direct contact approaches were not very practical from a commercial viewpoint. One problem was that the contact means were subject to sensing tolerances beyond what is required in the present day impact printer field. This may have been due in part to the indefiniteness of the exact point of impact which could be sensed by contact means. This was due in part to the initial contact which must be made with the print wheel and the ribbon before contact is made with the platen.
Further developments in means for sensing impact printer hammer flight and velocity covered in copending application R. H. Sweat, Jr., et al., Ser. No. 80,890 filed Oct. 1, 1979, the details of which are incorporated by reference into the description of the embodiment of the present invention, relate to means for determining the flight time of the impelled impact means by sensing velocity changes in the impact means and means responsive to the sensed flight time for controlling the impelling means to vary the energy pulse duration to the impact means. However, it has been discovered that varying the energy pulse duration to the impact means does not give consistent results. This is due to variations in the friction in impact missile bearings during free flight, after the energy pulse has ended. Therefore, the uncontrolled energy transfers during free flight of the impact missile, component tolerance variations among hammer missiles, and control circuit drift with age and temperature change result in variations in missile impact force and flight time that are not controllable by varying the energy pulse duration. Furthermore, in the Sweat, Jr. et al. application, the pulse duration is varied each time the actual impact time fails to match the predetermined impact time by a set amount. Varying the pulse duration in this fashion results in a compounding of the hammer missile to print element registration error due to the oscillation of the energy pulse width control circuit.